Lesson Notes By Weeks and Term v4 - SHS 3
ALTERNATING CURRENT
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
ALTERNATING CURRENT
Download the Lessonotes Mobile Ghana app for faster lesson access on Android and iPhone.
Subject: Physics
Class: SHS 3
Term: 2nd Term
Week: 8
Grade code: 3.3.2.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 3.3.2.CS.2
Indicator code: 3.3.2.LI.2
Theme: ELECTRIC FIELD, MAGNETIC FIELD AND ELECTRONICS
Subtheme: ALTERNATING CURRENT
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
This lesson introduces two fundamental concepts in Alternating Current (AC) circuits: reactance and impedance. Unlike Direct Current (DC), where opposition to current is simply resistance, AC circuits containing components like coils (inductors) and capacitors exhibit a more complex, frequency-dependent opposition. Understanding this is crucial because the electricity supplied by the Electricity Company of Ghana (ECG) to our homes, schools, and industries is AC. This knowledge helps explain how devices from simple ceiling fans to complex radio tuners work.
A. Recap: Resistance in AC circuits
In a pure resistor connected to an AC source, the opposition to the current flow is called resistance (R), measured in ohms (Ω). The key characteristic is that the voltage across the resistor and the current through it are in phase. This means they reach their maximum and minimum values at the same time. For AC circuits, Ohm's law is applied using RMS (Root Mean Square) values: `V_rms = I_rms * R`. B. Reactance (X)
Reactance is the opposition to the flow of alternating current caused by inductors and capacitors. It is also measured in ohms (Ω). Unlike resistance, reactance is frequency-dependent. Inductive Reactance (X_L) Concept: An inductor (a coil of wire, like in a motor or a fan) opposes any *change* in current. Since AC is constantly changing direction and magnitude, an inductor continuously opposes it. This opposition is called inductive reactance. Dependence: The opposition is greater if the current changes faster (higher frequency, *f*) or if the inductor is more powerful (higher inductance, *L*). Formula: The formula for inductive reactance is: ``` X_L = 2πfL ``` Where: `X_L` = Inductive Reactance (in Ω) `f` = Frequency of the AC supply (in Hertz, Hz). For ECG in Ghana, f = 50 Hz. `L` = Inductance of the coil (in Henry, H). `2π` comes from the angular frequency (ω = 2πf). So, `X_L = ωL`. Phase Relationship: In a pure inductor, the voltage across it leads the current by 90° (or π/2 radians). A helpful mnemonic is "ELI": in an inductor (L), voltage (E) comes before current (I).